The invention relates to a method for data transmission in a time division multiple access (TDMA) mobile communication system, comprising the step of assigning one or more time-slots to a mobile station for data transmission depending on the data transfer rate required by an application using the mobile station.
In mobile telecommunication systems of the time division multiple access (TDMA) type, communication takes place on the radio path in successive TDMA frames, each of which consists of several time-slots. In each time-slot, a short information packet is sent in form of a radio frequency burst which has a finite duration and which consists of a set of modulated bits. The timeslots are mainly used for conveying control channels and traffic channels. On the traffic channels, speech and data are transmitted. On the control channels, signalling between a base station and mobile subscriber stations is carried out. An example of a TDMA radio system is the Pan-European mobile communication system GSM (Global System for Mobile Communications).
For communication in conventional TDMA systems, each mobile station is assigned one channel time-slot for data or speech transmission. Thus, the GSM system, for instance, may have up to eight parallel connections to different mobile stations on a same carrier frequency. The maximum data transfer rate on one traffic channel is restricted to a relatively low level depending on the available channel bandwidth as well as the channel coding and error correction used in the transmission. In the GSM system, for instance, the user rate is 9.6 kbit/s or 12 kbit/s. In addition, in the GSM system a so-called half-rate (max. 4.8 kbit/s) traffic channel may be chosen for low speech coding rates. A half-rate channel is established when a mobile station communicates in a time-slot only in every second timeslot, i.e. at half-rate. A second mobile station communicates in every second time-slot of the same frame. The capacity of the system in terms of the number of subscribers may thus be doubled, in other words, up to 16 mobile stations may communicate simultaneously on the same carrier frequency.
In the last few years, the need for high-speed data services in mobile communication networks has remarkably increased. Data transfer rates of at least 64 kbit/s would be required for utilizing ISDN (Integrated Services Digital Network) circuit switched digital data services, for example. Data services of the public network PSTN, such as modems and telefax terminals of class G3, require higher transfer rates, such as 14.4 kbit/s. One of the growing areas of mobile data transfer that requires higher transfer rates than 9.6 kbit/s is the mobile video service. Services of this kind include e.g. security control by cameras, and video databases. The minimum data transfer rate in video transfer may be, for instance, 16 or 32 kbit/s.
The data transfer rates of the present mobile communication networks are not, however, adequate to satisfy these new needs.
One way to solve the problem is to use multiple TDMA time-slots for communication with a mobile station. In this way, one mobile station may transmit and receive data at higher data transfer rates by multiplexing the data to multiple time-slots (traffic channels). This is termed as multi-slot access technique.
When multiple time-slots are used for communication with one mobile station, problems arise if a sufficient number of traffic channels in not available. This may occur in the call set-up phase or in a handover. Handover refers to switching the mobile station from one channel onto another channel of the same cell or an adjacent cell during an ongoing call. A problem arises if the mobile station is operating at a high data transfer rate, and the new cell can not provide after the handover a data transfer rate as high as that of the previous cell. One solution would be to interrupt the connection if the quality i.e. the data transfer rate of the data transfer service is not adequate in the call set-up or handover, or after the handover. However, this solution is not acceptable.
An object of the present invention is to relieve the problems caused by capacity restrictions in a mobile communication network which employs the multi-slot access technique in data transmission.
This is achieved with the method for data transmission in a time division multiple access (TDMA) mobile communication system, comprising the step of assigning one or more time-slots to a mobile station for data transmission depending on the data transfer rate required by an application using the mobile station. The method is characterized in that the mobile station indicates to the mobile communication network the minimum and maximum requirements for the user data transfer rate, and that the mobile communication network assigns the mobile station for a data call a channel configuration consisting of one or more time-slots in connection with call set-up or handover. Said channel configuration depends on the network resources currently available in the mobile communication network and enables the data channel performance which is not lower than said minimum requirement and not higher than said maximum requirement. The serving mobile communication network will interrupt a data call set-up or a handover attempt of a data call if the mobile communication network has not sufficient network resources for providing the minimum requirement of the mobile station for the user data transfer rate. Said resources of the mobile communication network, depending of which the channel configuration determining the user data transfer rate is assigned to a data call, include at least the time-slot resources of the serving cell and the timeslot resources of the target cell for handover.
In the invention, the data transfer rate of a data call is reduced in a mobile communication system employing a so-called multi-slot access technique when a channel configuration that meets the maximum requirements for the user data transfer rate is not available. This may take place during call set-up or handover. The mobile station may start a high-speed data connection by transmitting the serving mobile communication network the minimum and the maximum requirements for the user data transfer rate, in addition to the presently specified parameters used for establishing a data call. These requirements may include the following parameters: the required level of service (data transfer rate) and the desired level of service (data transfer rate). The desired level of service determines the data transfer rate the mobile station wishes to be able to use. Simultaneously, the desired data transfer rate is the maximum data transfer rate to be allowed for the mobile station. With this parameter, the user of the mobile station may, e.g. call-specifically, in each case choose the most appropriate data transfer service in terms of speed and costs. The required level of service determines the minimum data transfer rate that must be provided for ensuring the continuity of data transfer. If the required data transfer rate cannot be provided, the call set-up is interrupted or the data call is interrupted. With this parameter, the user of the mobile station may, e.g. call-specifically, choose the lowest data transfer rate that in each case is sufficient for data transfer purposes. The parameters of the invention thus provide flexibility to the subscriber of the mobile station in the selection of the data transfer service. Alternatively, the parameters may be permanently set at the mobile station, or the mobile station may select the appropriate parameters. These parameters, i.e. the required and the desired level of service, allow the mobile communication network to vary the data transfer rates of individual mobile stations in accordance with the needs and the traffic load of the network, within the limits set by the parameters, without causing any disturbance to the users. In particular, these parameters allow the mobile communication network to reduce the data transfer rate if the new cell is not capable of providing the desired data transfer rate in connection with call set-up or handover. Thus, the number of calls interrupted or prevented due to the lack of resources decreases.
Instead of transferring the parameters, the mobile station may indicate said minimum and maximum requirements to the mobile communication network in a number of alternative ways, such as indicating the level of service. The minimum and the maximum requirements used are then selected in accordance with the indicated level of service in the mobile communication network.
It is also possible that the mobile station does not set any requirements for the minimum data transfer rate, or it leaves the minimum performance to be freely chosen by the mobile communication network.
As a result, the service of the data call may be continued in the new cell after the handover, totally regardless of how the data call was served in the previous cell, or independently of the service provided by the new cell.
The invention also relates to a mobile communication system for implementing the method.